As a conventional method of manufacturing a polyacetal (POM) copolymer, the cation copolymerization is known in which trioxane and a cyclic ether and/or cyclic formal having at least one carbon-carbon bond are used as a main monomer and a comonomer, respectively. Cationic active catalysts used for the above copolymerization may include Lewis acid, in particular, halides of boron, tin, titanium, phosphorus, arsenic and antimony, for example, boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorus pentachloride, phosphorus pentafluoride, arsenic pentafluoride and antimony pentafluoride, and compounds such as complexes or salts thereof: protonic acid, for example, perchloric acid; esters of protonic acid, in particular, esters of perchloric acid and lower aliphatic alcohol, for example, tert-butyl perchlorate ester; anhydrides of protonic acid, in particular, mixed anhydrides of perchloric acid and lower aliphatic carboxylic acid, for example, acetyl perchlorate, or trimethyloxonium hexafluorophosphate, triphenyl-methylhexafluoro alzenate, acetyl tetrafluoroborate, acetyl hexafluorophosphate, acetyl hexafluoro alzenate and the like. Among these, boron trifluoride, or coordination complexes of boron trifluoride and organic compounds, for example, ethers are the most common catalysts for polymerization in which trioxane is used as a main monomer, and they are widely used in industries.
However, in the case of the commonly used polymerization catalysts such as boron trifluoride-based compounds, a relatively large amount of catalyst (for example, 40 ppm or more relative to the total monomers) is required for polymerization. Therefore, sufficient catalyst deactivation treatment may not easily be performed after polymerization. Further, even in a case where a catalyst is successfully deactivated, substances from the catalyst may remain in a copolymer, causing problems such as promoted decomposition of the copolymer. Moreover, the catalyst deactivation process is usually performed in a large amount of an aqueous solution containing a basic compound such as triethylamine, and requires complicated steps such as separating a copolymer from a process liquid for drying after catalyst deactivation and collecting unreacted monomers dissolved in the process liquid, which are economically disadvantageous.
In order to avoid complicated steps associated with the catalyst deactivation treatment described above, proposed are methods comprising adding a trivalent phosphorus compound to a copolymer produced (for example, see Patent Document 1 and the like) and methods comprising adding a hindered amine compound (see Patent Document 2 and the like). Nonetheless, satisfactory effects have not been obtained.
Meanwhile, methods of manufacturing a polyacetal copolymer in which a heteropolyacid is used as a catalyst have been proposed (for example, see Patent Document 3 and the like). Further, methods of manufacturing a polyacetal copolymer have been proposed, the methods comprising: preparing a crude polyacetal copolymer by performing copolymerization using a heteropolyacid as a catalyst; then adding at least one solid basic compound selected from triazine ring-containing compounds and polyamides having amino groups or substituted amino groups to the reaction product; and performing melt-kneading treatment to deactivate the catalyst (for example, see Patent Document 4 and the like). According to the above methods, polymerization can be achieved with a very small catalytic amount because a heteropolyacid is highly active, and thus can provide a high quality polyacetal copolymer. Further, complicated steps as described above are not required because catalyst deactivation can be performed by melt-kneading treatment with essentially no solution, showing an economical advantage.
Patent Document 1: Japanese Examined Patent Application Publication No. S55-42085
Patent Document 2: Japanese Unexamined Patent Application Publication No. S62-257922
Patent Document 3: Japanese Unexamined Patent Application Publication No. H01-170610
Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2003-026746